A conventional automatic transmission mounted on an automotive vehicle comprises a transmission mechanism having a drive force from an engine transmitted thereto through a torque converter. The transmission mechanism has a plurality of clutches and brakes each serving as a friction engagement element, and a plurality of gears constituting power transmission paths selectively changed in response to the selective engagement and disengagement of the clutches and brakes to form a desired speed change stage.
The clutches and the brakes are changed into either one of their engagement states and disengagement states from the other of their engagement states and disengagement states with an oil pressure circuit changed in its states by solenoids and manual valves forming part of the oil pressure control circuit, thereby making it possible to establish a plurality of forward movement speed change stages and a rearward movement speed change stage in response to the operation position of a shift lever.
The manual valve is constituted to be mechanically changed in its states in response to the operation positions of the shift lever, while an oil pump driven by the engine is operative to produce a line pressure to be supplied to each of the oil pressure circuits.
In the recent years, there has been proposed a shift-by-wire type of control system, which is electrically controlled by the shift lever in response to the operation positions of the shift lever. With the shift-by-wire type of control system emerging as above, there has also been developed an automatic transmission with no manual valve as disclosed by the Japanese Publication No. 2002-533631 (Patent Publication No. 1).
The conventional automatic transmission as disclosed by the Japanese Publication No. 2002-533631 is operated with an electromagnetic valve logic that is based to selectively operate either one of the solenoid valves, due to the fact that the automatic transmission is equipped with no manual valve. This means that either one of solenoid valves is selected to be operated to establish the rearward movement speed change stage or the desired forward movement speed change stage. The conventional automatic transmission as disclosed by the Japanese Publication No. 2002-533631 is thus operated with the selected solenoid valves to operate the brakes and the clutches selected on the basis of a clutch logic so that the rearward movement speed change stage or the desired forward movement speed change stage can be established.
Here, the automatic transmission realized to change the shift range with the shift-by-wire type of control system comprises shift position sensors corresponding to the operation positions of the shift lever, the shift position sensors being operative to output information indicative of the detection results based to have the transmission control apparatus (hereinafter simply referred to as “T-ECU”) control each of the solenoid valves.
Here, the T-ECU is necessary to hold information indicative of either one of the shift ranges actually selected from among the shift ranges for determining the failure of the automatic transmission.